A laser diode includes a p-n junction between a pair of mirrors for creating optical feedback for light generated and amplified at the p-n junction when a forward current is applied to the p-n junction. To provide wavelength tenability, the mirrors are made wavelength selective, and a reflection wavelength of at least one of the mirrors is tuned.
In waveguide laser diodes, waveguide gratings are frequently used as wavelength selective mirrors. In a waveguide grating, periodic perturbations of the effective refractive index of the waveguide are created to selectively reflect light at a wavelength corresponding to the spatial frequency of the periodic refractive index perturbations. A waveguide grating can be tuned by heating or, for waveguide gratings formed at a p-n junction, by providing a direct current to the p-n junction, which changes its overall refractive index through carrier injection.
Current-tunable p-n junction waveguide gratings have drawbacks. Supplying direct current to a waveguide grating can induce optical loss, which negatively impacts laser light generation efficiency and broadens the emission spectral linewidth of the laser. Thermally tuned gratings are generally free from these drawbacks. However, thermal tuning requires considerable amounts of heat applied to the waveguide grating to change its temperature, which can also impact the temperature of the lasing p-n junction. This is because waveguide gratings are typically fabricated integrated with the lasing p-n junction, which must be heat sunk very well to prevent overheating of the laser diode during normal operation. By way of example, Ishii et al. in an article entitled “Narrow spectral linewidth under wavelength tuning in thermally tunable super-structure grating (SSG) DBR lasers”, published in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2 (1995), pp. 401-407, disclose a super-structure grating distributed Bragg reflector laser, which can be thermally tuned over 40 nm by thermally tuning SSG reflectors. In the Ishii device, the max thermal tuning power dissipation per unit length of mirror to achieve full tenability was 1.3 mW per 1 micrometer of length, which for the front and back mirror section lengths used of 400 and 600 micrometers, respectively, corresponds to a prohibitively-high total power dissipation of 1300 mW. The tuning 1/e time constant is about 1.6 milliseconds, which is relatively slow.
Attempts have been made in the prior art to utilize thermal tuning more efficiently by thermally decoupling the waveguide grating from the common substrate with the lasing p-n junction. By way of example, Cunningham et al. in U.S. Pat. No. 7,848,599 disclose a thermally tunable waveguide that is free standing above a substrate to increase thermal resistance between the waveguide and the environment. Matsui et al. in U.S. Pat. No. 7,778,295 disclose a Distributed Bragg Reflector (DBR) laser, in which the DBR section of the laser is suspended over the substrate to increase the thermal resistance between the DBR section and the substrate.
Detrimentally, waveguides suspended over a substrate without additional structural support are prone to a mechanical failure. Multiple legs were used in a Cunningham device to support the suspended waveguide along their length, but these can result in an overly complex waveguide structure and/or interfere with the optical function of the waveguide.
It is therefore a goal of the invention to provide a tunable waveguide grating that could be tuned quickly and efficiently, substantially without degradation of spectral properties, while providing an adequate structural support for the waveguide.